LT3479 LINER [Linear Technology], LT3479 Datasheet - Page 9

• Manufacturer Part Number: LT3479
• Description: 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast switching speed, are ideal for LT3479 applications.
Table 3 lists several Schottky diodes that work well with the
LT3479. The diode’s average current rating must exceed
the average output current. The diode’s maximum reverse
voltage must exceed the output voltage. The diode conducts
current only when the power switch is turned off (typically
less than 50% duty cycle), so a 3A diode is suffi cient for
most designs. The companies below also offer Schottky
diodes with high voltage and current ratings.
Table 3. Suggested Diodes
MANUFACTURER
PART NUMBER
UPS340
UPS315
B220
B230
B240
B320
B330
B340
SBM340
Setting Positive Output Voltages
To set a positive output voltage, select the values of R1 and
R2 (see Figure 2) according to the following equation:
V
Figure 2. Positive Output Voltage Feedback Connections
OUT
=
1 235 1
.
CURRENT (A)
V
MAX
⎛
⎜
⎝
3
3
2
2
2
3
3
3
3
LT3479
+
R
R
2
1
V
⎞
⎟
⎠
FBN
FBP
MAX REVERSE
REF
VOLTAGE (V)
40
15
20
30
40
20
30
40
40
V
OUT
R1
R2
3479 F02
MANUFACTURER
Microsemi
www.microsemi.com
Diodes, Inc
www.diodes.com
Setting Negative Output Voltages
To set a negative output voltage, select the values of R3 and
R4 (see Figure 3) according to the following equation:
Board Layout
As with all switching regulators, careful attention must
be paid to the PCB board layout and component place-
ment. To maximize effi ciency, switch rise and fall times
are made as short as possible. To prevent radiation and
high frequency resonance problems, proper layout of the
high frequency switching path is essential. Minimize the
length and area of all traces connected to the SW pin and
always use a ground plane under the switching regulator
to minimize interplane coupling. The signal path including
the switch, output diode D1 and output capacitor C
contains nanosecond rise and fall times and should be
kept as short as possible. Recommended component
placement is shown in Figure 4.
Soft-Start
For many applications, it is necessary to minimize the
inrush current at start-up. The built-in soft-start circuit
signifi cantly reduces the start-up current spike and output
voltage overshoot. A typical value is 10nF for 1.65ms.
Figure 5 shows the start-up output voltage and induc-
tor current waveforms in a typical application without a
soft-start capacitor. Notice the output voltage overshoot
and the large initial current. The addition of a 22nF capaci-
tor eliminates the output overshoot and reduces the peak
inductor current (Figure 6).
V
Figure 3. Negative Output Voltage Feedback Connections
OUT
=
– . 1 235
V
⎛
⎜
⎝
R
R
LT3479
3
4
⎞
⎟
⎠
V
FBN
FBP
REF
–V
OUT
R3
R4
3479 F03
LT3479
OUT
3479fb
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